Pachymetry Test: Purpose, Test Details & Results

Pachymetry is a specialized eye test that measures the thickness of the cornea—the transparent, dome-shaped surface that covers the front of the eye. As a non-invasive and widely-used diagnostic tool, pachymetry plays a crucial role in eye care, especially in diagnosing and managing diseases like glaucoma, keratoconus, and in assessing suitability for refractive surgery. Understanding why, how, and what pachymetry reveals can empower patients and guide clinicians in making optimal decisions for eye health.

Purpose of Pachymetry Test

A pachymetry test is more than just a measurement—it's a gateway to understanding the health of your eyes. This section explores the main reasons doctors recommend pachymetry and the clinical decisions that hinge on accurate corneal thickness data.

Understanding the Need for Pachymetry

Detecting and Managing Glaucoma

Glaucoma is a major cause of blindness worldwide. Accurate measurement of intraocular pressure (IOP) is essential for diagnosis and management. However, corneal thickness directly influences IOP readings—a thin cornea can cause underestimation, while a thick cornea can lead to overestimation. Pachymetry helps correct for these variations, ensuring a more precise assessment of glaucoma risk and progression 3 4 8.

Screening and Monitoring Keratoconus

Keratoconus is a degenerative disease where the cornea thins and bulges outward. Early detection is critical for interventions that can halt or slow progression. Pachymetry maps can identify abnormal patterns of thinning and asymmetry, even in subclinical cases where other signs may be subtle or absent 5 6. This is especially valuable for screening at-risk individuals and monitoring disease evolution.

Assessing Suitability for Refractive Surgery

Procedures like LASIK and PRK reshape the cornea to correct vision. If the cornea is too thin, surgery can be unsafe, risking structural instability and complications like ectasia. Pachymetry confirms whether a patient meets the minimum safety threshold for surgery and helps in surgical planning 4 9 11.

Monitoring Corneal Diseases and Post-Operative Healing

Beyond diagnosis, pachymetry is vital for tracking disease progression in conditions like Fuchs endothelial corneal dystrophy (FECD) and post-surgical healing. Changes in corneal thickness can signal worsening disease or complications, guiding further treatment 10 12.

Pachymetry Test Details

The process of pachymetry is high-tech yet patient-friendly. This section breaks down the technologies involved, what patients can expect during the test, and the reliability of various methods.

Technologies Used in Pachymetry

Ultrasound Pachymetry

This is the most traditional and widely used method. A handheld probe touches the numbed surface of the cornea and emits ultrasonic waves, which reflect back to the device. The time taken for the echo to return is used to calculate corneal thickness. Ultrasound pachymetry is known for its within-session repeatability and reliability, making it a standard in many clinics 1 4 8 9.

Pros:

• Accurate and repeatable
• Cost-effective
• Portable

Cons:

• Requires topical anesthesia
• Contact method carries minimal infection risk
• Technique-sensitive

Optical Coherence Tomography (OCT)

OCT uses light waves to capture cross-sectional images of the cornea. It's completely non-contact, which enhances comfort and eliminates infection risk. OCT can provide detailed pachymetric maps, showing thickness at multiple corneal points, not just the center. It is particularly valuable for mapping irregularities in diseases like keratoconus 3 7 8 9.

Pros:

• Non-contact and comfortable
• Highly repeatable, especially in between-session tests
• Excellent for detailed corneal mapping

Cons:

• Slightly different readings compared to ultrasound
• More expensive

Scanning-Slit and Specular Microscopy

These optical methods use slit-scanning or microscopic imaging to assess corneal thickness. They are useful for not only measuring thickness but also evaluating corneal shape and endothelial cell health. However, readings may be systematically higher or lower than those from ultrasound, so devices are not interchangeable. For consistency in follow-up, the same instrument should be used 1 2 4.

Scheimpflug Imaging

Scheimpflug systems use a rotating camera to capture multiple images across the cornea and reconstruct a detailed three-dimensional map. This method is non-contact, highly repeatable, and can also predict the prognosis of corneal diseases like FECD. However, measurement agreement with other devices (like OCT) can vary, so results should be interpreted in context 7 10 11.

What to Expect During the Test

• Preparation: For contact methods, numbing eye drops are applied.
• Procedure: The patient is seated at an instrument, asked to fixate on a target, and measurements are taken in seconds.
• Comfort: Most methods are painless and quick, with minimal discomfort for contact techniques.
• Aftercare: There are no restrictions post-test; normal activities can be resumed immediately.

Comparing Reliability and Repeatability

• Ultrasound: Best for within-session repeatability 8.
• OCT: Best for between-session repeatability and detailed mapping 8.
• Scheimpflug and other optical methods: Highly repeatable, but differences between devices mean one system should be used consistently for monitoring 1 7 8.

Pachymetry Test Results & Follow-Up

Understanding pachymetry results is essential for both patients and clinicians. This section explains what the numbers mean, how they impact care, and what might come next based on the findings.

What Do Pachymetry Results Mean?

Normal Corneal Thickness

• Average Range: Typically 520–560 micrometers (μm) for healthy adults, but this can vary by population and measurement device 3 4 8 9.
• Implications: Indicates low risk for corneal diseases; suitable for most refractive surgeries.

Thin Cornea

• Implications: Greater risk for glaucoma due to the potential underestimation of IOP; may indicate keratoconus or other thinning disorders 5 6 10.
• Response: Doctor may order additional imaging, genetic testing, or recommend more frequent monitoring. Refractive surgery may be contraindicated.

Thick Cornea

• Implications: Can lead to overestimation of IOP, possibly masking glaucoma. May also affect eligibility for some surgeries 3 8 9.
• Response: Clinicians adjust risk calculations and may use alternative approaches for diagnosis or monitoring.

Abnormal Pachymetric Maps

Modern pachymetry can generate detailed thickness maps, not just a single central reading. Abnormal patterns—such as focal thinning, asymmetry, or displacement of the thinnest point—can signal early or subclinical disease even when overall thickness is within normal limits 5 6 10. In conditions like FECD, specific map features (like loss of regular isopachs or displaced thinnest point) can predict prognosis independent of central thickness 10.

Follow-Up Actions

• No Abnormality: Routine eye exams.
• Suspicious Results: Additional imaging, more frequent monitoring, or referral to a corneal specialist.
• Post-Surgery: Regular pachymetry to monitor healing, detect complications, or assess treatment efficacy 4 11 12.
• Disease Monitoring: Ongoing serial pachymetry to track progression or response to therapy.

Interpreting Variations Between Devices

It's crucial for both patients and providers to understand that measurements can differ between devices and methods. The same instrument should be used for ongoing monitoring to ensure consistency 1 2 3 7 9. Small differences between sessions are normal, but significant changes may warrant closer attention.

Conclusion

Pachymetry is a powerful, non-invasive test that offers insights into the health and structure of the eye's cornea. Its applications range from glaucoma risk assessment to surgical planning and disease monitoring. Understanding the nuances of test methods and the significance of results helps patients and providers collaborate for optimal eye health.

Key takeaways:

• Pachymetry is essential for glaucoma management, keratoconus detection, refractive surgery planning, and tracking corneal diseases 3 4 5 6 9 10.
• Multiple technologies exist (ultrasound, OCT, Scheimpflug, etc.), each with strengths and limitations. Consistency in device use is important for monitoring 1 3 4 7 8 9.
• Test results guide care: Thin, thick, or abnormal corneal findings can influence diagnosis, prognosis, and treatment decisions 3 5 6 10.
• Most tests are quick, painless, and safe; results are immediately useful for clinical decisions.
• Regular follow-up may be necessary if abnormalities are detected, and the same testing method should be used over time 1 2 3 7 9.

Empowered with this knowledge, patients can better understand their eye health and the pivotal role pachymetry plays in modern ophthalmology.